Equuleus (“little horse”) is a faint northern sky constellation that was included by the Greek astronomer Ptolemy in his astronomy treatise called Almagest around 150 AD. It is the second smallest of the night sky’s 88 recognized constellations, with its brightest star, Kitalpha, a yellow-white giant found 186 light years away that shines with an apparent magnitude of just 3.92.
Equuleus can be seen by observers located between +90° and -80° of latitude, although best seen from northern latitudes during September and October. This twisted rectangular shaped constellation can be found just west of the head of Pegasus, with its bordering constellations also including Aquarius and Delphinus.
Heavenly Waters Family
Equuleus is a member of the Heavenly Waters family of constellations, along with Carina, Columba, Delphinus, Eridanus, Piscis Austrinus, Puppis, Pyxis, and Vela.
Equuleus is associated with Celeris, a young horse and brother to the famed winged horse Pegasus that was given to Castor by the god Mercury. Because it rises in the night sky before Pegasus, the constellation was often referred to as Equus Primus, meaning the First Horse. In another legend, however, it was said to represent Cyllarus, a magnificent horse who was gifted to Pollux by the goddess Hera, who in turn had received it from Poseidon.
Still another myth identifies the constellation with Hippe or Euippe, daughter of the centaur Chiron, who found herself seduced and impregnated by Aeolus, the wind god. Unwilling to tell her father the embarrassing truth, she kept her pregnancy a secret and eventually ran off to the mountains where she gave birth to Melanippe. After her father came looking for her, Hippe prayed to the gods that her father would not find her, and so Artemis turned her into a mare, later placing her amongst the stars as a constellation. Even now Hippe appears to be hiding from her father, Chiron, as only the mare’s head is showing behind Pegasus in the night sky. Her father is represented by the Centaurus constellation.
– Kitalpha (Alpha Equulei), the constellation’s brightest star, is a spectroscopic binary star situated 186 light years from the solar system with an apparent visual magnitude of 3.92. Its primary component is a yellow-white giant (G0III) that is around 9 times bigger than our sun, 3 times more massive, and 50 times brighter. Its companion is an A-type dwarf, with the two stars having an orbital period of 98.8 days. The name Kitalpha derives from the Arabic phrase for “a piece of the horse.”
– Pherasauval (Delta Equulei), the second brightest star in Equuleus, is a binary star located 60 light-years away from our solar system that has a magnitude of 4.47. The system consists of a blue-white dwarf star and a yellow star that orbit each other once every 5.7 years. Pherasauval derives from the Arabic phrase maeaning “the First Horse.”
– Gamma Equulei, the third brightest star in Equuleus, is a binary star located 118 light-years away that has a visual magnitude of 4.7. It consists of a white star with an A2 dwarf companion separated by 0.18 light years. It primary component is also a strange star, chemically speaking, which undergoes sporadic variations in its brightness ranging from 4.58 to 4.77 over just a 12½ minute period.
Other stars of interest in Equuleus includes the blue dwarf stars HIP 103652 and 6 Equulei; the orange stars Zeta Equulei and HD 200964; the multiple star systems of Lambda Equulei, 4 Equulei, and Epsilon Equulei.
Notable Deep Sky Objects
There are no Messier objects in Equuleus, and being so small it also possesses no notable deep sky objects. It does, however, contain a number of dim galaxies ranging between magnitudes of 13 and 15, including the spiral galaxy NGC 7040; and the barred spiral galaxies NGC 7046 and NGC 7015.